Oyster settlement is enhanced by confamilial cues, substrate complexity and orientation: A hatchery experiment

Biogenic oyster reefs are important structures in marine ecosystems, boosting biodiversity and providing many ecosystem services. However, they have undergone global degradation in recent decades with limited success in restoration. A key bottleneck in oyster restoration is the absence of a suitable settlement substrate. Accordingly, new restoration techniques are needed that provide adequate settlement material for target oyster species. Under hatchery conditions, we experimentally investigated the effects of settlement cues and substrate complexity on the settlement of larvae of the European flat oyster (Ostrea edulis) and of the Pacific oyster (Crassostrea gigas). We found that the presence of fresh shell material of both oyster species in a lime-pozzolan concrete-based substrate positively affected larval settlement of both oyster species compared to substrates containing fresh cockle (Cerastoderma edule) shell material. This highlights a family-specific but not species-specific effect of signalling cues provided by the shell material. Indeed, substrates with oyster shell material deprived of settlement cues by high-temperature exposure (350 °C for four hours), also showed lower settlement than substrates containing fresh oyster shell material. We also found that increased substrate surface complexity boosted Pacific oyster larval settlement, and that oyster larvae of both species preferentially settled on the bottom of horizontal substrate surfaces. Overall, our results show the importance of cue specificity, the complexity of the settlement substrate, and the orientation of the attachment surface. These findings can be used to improve restoration efforts of oyster species in their natural and human-engineered habitats worldwide.